ChromeOS: Make OOBE wallpaper shield match login wallpaper shield.

Bug: 736903
Change-Id: Ic71d4c4d52f4ad17be6b8e9f251d79b534dea341
Reviewed-on: https://chromium-review.googlesource.com/579168Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488382}

ui/login/screen_container.css

@@ -68,7 +68,7 @@
 }
 
 #oobe-shield[md-mode] {
-  background-color: rgba(20, 29, 40, .8); /* #141D28 80% */
+  background-color: rgba(0, 0, 0, 0.6);
   display: block;
   height: 100%;
   left: 0;